The present invention relates to an induction device, such as a reactor or transformer, having a plurality of phases.
The invention is particularly applicable to a large reactor for use in a power system, for example in order to compensate for the Ferranti effect in long overhead lines or extended cable systems causing high voltages under open circuit or lightly loaded conditions. Reactors are sometimes required to provide stability to long line systems. They may also be used for voltage control and switched into and out of the system during lightly loaded conditions.
Transformers are used in power systems to step up and step down voltages to useful levels.
A typical known induction device comprises one or more coils wrapped around a laminated core to form windings, which may be coupled to the line or load and switched in and out of the circuit. The equivalent magnetic circuit of a static induction device comprises a source of magnetomotive force, which is a function of the number of turns in the winding, in series with the reluctance of the core, which may include iron and optionally an air gap.
The air gap represents a weak link in the structure of the core, which tends to vibrate at a frequency twice that of the alternating input current. This is a source of vibrational noise and high mechanical stress. Another problem associated with the air gap is that the magnetic field fringes, spreads out and is less confined. Thus, field lines tend to enter and leave the core with a non-zero component transverse to the core laminations which can cause a concentration in unwanted eddy currants and hot spots in the core.
It is known to alleviate these problems by placing one or more inserts in the air gap, for example comprising radially laminated steel plates and ceramic spacers. However, such inserts are complicated and difficult to manufacture and are therefore expensive.
It is known to provide a plurality of windings of different phases on a transformer or solenoid having a yoke similar to the stator of an asynchronous machine. See A. A. Martynov and V. V. Krushchev, “The Inductive Reactance of a Rotating Magnetic Field Multiphase Transformer with Yoke Magnetization”, Electrical Technology, No. 2, pp 39-47, 1994.
Preferably, the device of the invention is a high voltage device. In this specification, the term “high voltage” is intended to mean in excess of 2 kV and preferably in excess of 10 kV. The invention also relates to a method of regulating a high voltage induction device.
In WO-A-99/17315 there is disclosed an arrangement for regulating an induced voltage in a transformer or regulating the reactive power of a reactor. In this known arrangement the transformer/reactor has a flux carrier about which is arranged a regulating winding. The number of turns of the regulating winding arranged around the flux carrier can be adjusted to alter the electrical properties of the transformer/reactor.